Most analytical procedures for the determination of trace elements in biological and other organic materials require that the organic matrix be completely destroyed. Both wet and dry ashing procedures are commonly used to destroy organic matrices. Dry ashing or furnace ashing is more often the chosen method simply as a matter of convenience. Problems commonly associated with dry ashing include loss of elements due to volatilization or bonding with the ashing container, the formation of ash residue which is difficult to dissolve, and the foaming of the sample. Wet ashing has been the chosen method with respect to volatile metals, such as, for example, Hg, or metals which form volatile salts when ignited in the presence of chlorides, such as, for example, As, Se, Cr, Fe, Sb, and Pb.
Furnace ignition or dry-ashing usually requires ashing acids, such as, for example, Mg(NO.sub.3).sub.2 or H.sub.2 SO.sub.2 which may introduce undesirable metal contaminants. Wet-ashing methods sometimes require catalysts, such as, for example, vanadium pentoxide (V.sub.2 O.sub.5) for mercury determination, and various metal salts, including those of Cu, Ag, Au, Co, V, Ni, Pd, and Fe for Kjeldahl wet-ashing catalysts when determining nitrogen presence in organic compounds. These catalysts would also be a source of contamination or interference if the corresponding elements were being determined.
Some wet-ashing procedures are permitted to be incomplete. For example, in a digestion procedure prior to mercury determination, the fatty acids from waxes, oils, and fats are not broken down by the digestion process. Instead, they are subsequently removed by filtering the digest so as to leave the Hg salts dissolved in the aqueous filtrate. Another procedure for Hg determination brings fatty acids into solution by hydrolyzing the organic matrix with concentrated alkali. Yet another procedures uses either 50% or 30% hydrogen peroxide to produce colorless, clear digest solutions. However, these apparently completed digest solutions contain soluble organic peroxides which darken in color, and/or precipitate organic colloid micelles as soon as the peroxides decompose or are reduced. Incomplete destruction of the matrix may thus result in colloidal substances and colored products of decomposition, which would cause interferences in the determinative step of most analyses. Such interferences include clogging of atomic emission and absorption instrumentation nebulizers, formation of explosive gas mixtures during neutron irradiation for neutron activation analyses (NAA), and masking of metal chelate absorbance bands during spectrophotometric determinations.
A fusion technique using NaNO.sub.3 and KNO.sub.3 as a eutectic mixture oxidizes all organics including polyethylene at 390.degree..+-.10.degree. within a few minutes. Although nitrate fusion is a rapid and effective means of matrix destruction, explosion hazards associated with the use of nitrate salts to destroy organic matrices have been reported.
A widely used wet-ashing process that completely destroys organic matter uses a combination of nitric, sulfuric, and potentially explosive perchloric acids. This method is effective and safe so as long as (1) the digest is not permitted to boil dry, leading to volatilization losses and formation of possible perchlorate esters which are spontaneously explosive; (2) digestion of samples having fats and oils contents greater than 50% is not attempted; and (3) the digestion is closely watched so as to prevent charring which can also lead to volatilization losses and/or possible explosion.